The present invention relates to a process for removing soluble fluoride from a waste solution containing such soluble fluoride. In one embodiment, the invention relates to a process for recovery of synthetic fluorspar-grade calcium fluoride from a fluoride-containing aqueous effluent using calcium sulfate to form the calcium fluoride. The present invention is particularly useful in removal of fluoride from an effluent originating from an alkylation process utilizing hydrogen fluoride (HF; hydrofluoric acid) as the catalyst. The calcium sulfate used in the inventive process may originate as a byproduct from a process for converting calcium fluoride into hydrogen fluoride. The fluorspar-grade calcium fluoride recovered from the inventive process may be used as feedstock in the hydrogen fluoride production process, which converts calcium fluoride into hydrogen fluoride. Thus, the present invention further relates to a continuous, closed-loop process recycling both fluoride and calcium.
Environmental laws and efficient operation of wastewater treatment plants require that fluoride be removed and/or recovered from wastewater prior to discharge into the environment. Increasingly, it is being recognized that economic concerns demand recycle of materials such as fluoride for which environmental concerns demand removal from waste streams. A number of processes have been used for removal and/or recovery of fluoride from wastewater. For example, U.S. Pat. No. 4,734,200 discloses a process for treating acidic process waste water containing SiF6xe2x88x922 and phosphorus-type contaminants, in which the waste water is treated with a strong base ion exchange resin to remove SiF6xe2x88x922.
Similarly, U.S. Pat. No. 4,965,061 discloses a process for producing hydrofluoric acid from waste water containing SiF6xe2x88x922, in which the SiF6xe2x88x922 is reacted with (NH4)2SO4 at a high pH to form (NH4)2SiF6, from which ammonia, Si(OH)4 and HF are eventually produced.
U.S. Pat. No. 5,215,632 discloses a method of removing fluoride and sulfate from an aqueous stream by adding calcium chloride and phosphate to form CaSO4 and a compound of calcium, fluoride and phosphate. The CaSO4 is removed as a precipitate, and carbonate is added to cause the precipitation of calcium fluoride.
A system for removal and recycle of fluoride is known as the HARDTAC process and is available from Modular Environmental Technologies, Inc. According to the company""s website, (www.modenvtech.com), the HARDTAC process uses calcium chloride or lime as a source of calcium for reaction with fluoride to form calcium fluoride, and may be applied to treatment of a neutralized hydrogen fluoride acid blowdown from a refinery.
A process used and licensed by Phillips for neutralization of hydrogen fluoride and removal of fluoride uses sodium hydroxide to neutralize refinery waste hydrogen fluoride, and soluble calcium chloride as the source of calcium for formation of CaF2. The process separates solid CaF2 for disposal as solid waste and discharges an effluent containing sodium chloride. Another known process uses KOH to neutralize HF, and uses lime (Ca(OH)2) as the source of calcium for reaction with fluoride to form CaF2 and regenerates KOH for recycling to the hydrogen fluoride neutralization step.
Hydrogen fluoride for use in refinery alkylation processes is usually made from high-grade fluorspar, a CaF2 mineral, in a process wherein the CaF2 is treated with concentrated sulfuric acid or oleum (H2SO4+SO3). The products of this reaction are HF and CaSO4. The by-product CaSO4 is generally put in a landfill, since it has previously had little or no economic value.
The hydrogen fluoride produced for use in refinery alkylation processes, as well as for other uses in the chemical processing industry, must be free of chloride contamination, due at least in part to stress corrosion cracking of steel which results from or is exacerbated by the presence of chloride. Hydrogen fluoride produced from a recovery process which utilizes calcium chloride is not suitable for such uses due to the unavoidable presence of chloride contamination in the CaF2. In the process using KOH to neutralize HF, the emphasis mainly has been on recovery and recycle of KOH, and the CaF2 is generally recovered as a gel, dewatered, solidified and discarded in a landfill.
Lower grade hydrogen fluoride, such as that obtained from recycling calcium fluoride obtained by using calcium chloride to precipitate fluoride and thus containing elevated levels of chloride, or which has a broad range of particle sizes and/or very small particle sizes, or which contains some excess organics, base and water, may be suitable for use in aluminum processing, but is not suitable for use in refinery alkylation processes, and for use in other areas of the chemical industry.
One aspect of the present invention is a process for removing soluble fluoride from a waste solution containing the soluble fluoride, the process including the steps of:
(A) mixing the waste solution with an aqueous slurry containing seed calcium fluoride particles to form a first aqueous composition;
(B) mixing the first aqueous composition with a calcium containing reagent to form a second aqueous composition;
(C) advancing the second aqueous composition through a tubular reactor for an effective period of time and at a velocity sufficient to permit the soluble fluoride and the calcium containing reagent to react and attach to the seed particles of calcium fluoride, the seed particles of calcium fluoride thereby increasing in size to become enhanced particles; and
(D) removing a portion of the particulate of the enhanced particles from said second aqueous composition.
Another aspect of the present invention is a process for removing soluble fluoride from a waste solution containing the soluble fluoride and manufacturing hydrogen fluoride from the fluoride so removed, the process including the steps of:
(A) mixing the waste solution with an aqueous slurry containing seed calcium fluoride particles to form a first aqueous composition;
(B) mixing the first aqueous composition with a calcium containing reagent to form a second aqueous composition;
(C) advancing the second aqueous composition through a tubular reactor for an effective period of time and at a velocity sufficient to permit the soluble fluoride and the calcium containing reagent to react and attach to the seed particles of calcium fluoride, the seed particles of calcium fluoride thereby increasing in size to become enhanced particles; and
(D) removing from the second aqueous composition substantially only enhanced particles having an average particle size of at least about 20 xcexcm;
(E) treating the enhanced particles removed in step (D) with sulfuric acid to form hydrogen fluoride and calcium sulfate.
In one embodiment, the present invention includes a process for removing soluble fluoride from a waste solution containing the soluble fluoride, the process including the steps of:
(A) mixing the waste solution with an aqueous slurry containing seed calcium fluoride particles to form a first aqueous composition;
(B) mixing the first aqueous composition with calcium sulfate to form a second aqueous composition;
(C) advancing the second aqueous composition through a tubular reactor for an effective period of time and at a velocity sufficient to permit the soluble fluoride and the calcium containing reagent to react and attach to the seed particles of calcium fluoride, the seed particles of calcium fluoride thereby increasing in size to become enhanced particles; and
(D) removing a portion of the enhanced particles from the second aqueous composition, wherein the portion contains substantially only particles having a size greater than about 20 xcexcm; and
(E) removing water from the second aqueous composition, wherein the water removed contains less than about 50 ppm of soluble fluoride.
Thus, the present invention provides a continuous process which enables the use of calcium sulfate tailings to recover fluoride from wastewater, wherein the wastewater is discharged from, e.g., a process which utilizes hydrogen fluoride, such as a catalytic refinery alkylation process. The inventive process may be employed to generate high grade CaF2 which can be used in the manufacture of hydrogen fluoride of sufficient purity for use in the alkylation process, and which contains substantially no chloride.